1. Field of the Invention
The invention relates to a combustion bowl in the crown of a piston for a diesel engine having a ring-shaped cooling channel disposed in the radially outer edge region of the piston on the piston crown side, whereby the combustion bowl as well as its bowl neck are disposed outside of the center relative to the longitudinal piston axis.
In a diesel engine with direct injection, combustion of the fuel/air mixture generally takes place in a combustion bowl that is formed into the crown of the piston of the diesel engine. It is known, in this connection, to provide the radially outer edge region of the combustion bowl with an undercut. A torque curve that is constant in a broad rpm range of the diesel engine in question was already mentioned as an advantage of this in the German patent document No. 721889. According to the German laying-open document 1055873, the cause of this lies in the swirling caused by the undercut, and an accompanying improvement in the evaporation of the fuel injected into the combustion bowl.
2. The Prior Art
A piston for a diesel engine is known from the European patent document EP 105 933, having a combustion bowl disposed outside of the center, into the radially outer edge region of which fuel is injected by way of several injection openings or fuel nozzle. To improve the swirling and therefore the evaporation of the fuel, the piston has a plurality of recesses uniformly distributed over the edge region.
The cited state of the art has the disadvantage that because of the combustion of the fuel/air mixture, the radially outer edge of the combustion bowl, in particular, is exposed to very great temperature stress, which can result in damage of the combustion bowl.
In the German utility model DE 80 26 159, it is suggested to provide a cooling channel between the edge of a combustion bowl disposed in the center, and the piston crown edge, in order to solve this problem. If the piston known from the prior art last mentioned is supposed to be used in a two-valve engine, in which the fuel nozzles is disposed next to the inlet and outlet valve lying symmetrical to the piston axis, i.e. outside of the center relative to the longitudinal piston axis, it is advantageous if the combustion bowl is also disposed outside of the center relative to the longitudinal piston axis, so that the fuel can be injected into the combustion bowl by the fuel nozzle, without problems.
In this connection, there is the problem that there is no room for a cooling channel between the top land of the piston and the edge region of the combustion bowl disposed outside of the center that lies closest to the top land, so that at least in this region, the cooling channel must be disposed farther away from the piston crown, in the boss region of the piston. This has the disadvantage that in this region, the edge of the combustion bowl, the bowl neck, and also the compression ring groove, which is subject to very great thermal stress, are cooled poorly and run the risk of being damaged. Another disadvantage in this connection is that the cooling channel, which furthermore lies very close to the piston crown, assumes a slanted position relative to the longitudinal piston axis, and this results in production technology problems.
Finally, the problem occurs, in this connection, that the distance between the part of the cooling channel disposed in the edge region of the piston crown and the edge region of the combustion bowl that is disposed outside of the center, which lies at a distance from the former, becomes so great that cooling of this edge region of the combustion bowl is also impaired.